I have kept mine in the bucket for almost a week, but you will get evaporation as well as a brown clay like sediment on the sides and bottom of the bucket. You should keep a powerhead and heater running for best results. Also, if your bucket is not food safe, the longer the water is in there the more phosphates, etc. will leach from the plastic. As letoan said, some people mix a big batch and use it all month long. If you store it properly, I don't think it goes bad.